SWIFTS spectrometer is already known for its exceptional compactness and robustness. Despite its ability to reach very high spectral resolution such as R<150000, its sensitivity is relatively poor because it exploits single mode waveguides. SWIFTS-LA ("LA" stands for "Large Aperture") is a new device belonging to the generation of Static Fourier imaging spectrometers dedicated to high spectral resolution measurements. Inspired from MICROSPOC and SWIFTS technologies, we will show how this new device exploits stationary waves in high refractive index materials to get a very small spectrometer with a very high angular acceptance. This spectrometer is intimately coupled to infrared or visible detectors making them very stable, compact and sensitive. We will present some results demonstrating preliminary performances and quality of signal reconstruction. Based on these results, we will show how an implementation of SWIFTS-LA can meet at least CARBONSAT specifications in just a few litres spacecraft and how these principles can be implemented for planetary mission imaging spectrometers.
Compact and hand-held spectrometers may be very interesting for the measurement of spectral signatures of chemicals or objects. To achieve this goal, ONERA and IPAG have developed a new on chip Fourier Transform Spectrometer operating in the visible spectral range with a high spectral resolution (near 2 cm-1), named visible HR SPOC (visible High Resolution Spectrometer On Chip). It is directly inspired from the MICROSPOC infrared spectrometer, studied at ONERA in the past years. This spectrometer is made of a stair-step two-wave interferometer directly glued on a CMOS detector making it a very compact prototype. After calibrating the optical path difference, measurements of experimental spectra are presented.
There is a need for compact, hand-held, spectrometers for the measurement of spectral signatures of chemicals or objects. To achieve this goal, a new concept of Fourier-transform interferometer (FTIR) directly integrated on the infrared focal plane array (FPA) has been developed at ONERA. The fundamental properties of this key element called MICROSPOC will be recalled and we will see how those properties can be exploited to get a snapshot, compact and cryogenic MWIR spectrometer. These design rules have been applied to develop a very compact device that combines the metrological properties of a FTIR-FPA of quantum HgCdTe technology with the radiometric performances of a last generation Sofradir detection block (Infrared Detector Dewar Cooler Assembly – IDDCA). The experimental performances of the prototype will be presented, in terms of spectral resolution, acquisition rate, dynamic range and noise equivalent spectral radiance. We will discuss at the end the potential of this technology to meet the requirements of different applications.
Infrared focal plane arrays (IRFPA) are widely used to perform high quality measurements such as spectrum acquisition at high rate, ballistic missile defense, gas detection, and hyperspectral imaging. For these applications, the fixed pattern noise represents one of the major limiting factors of the array performance. This sensor imperfection refers to the nonuniformity between pixels, and is partially caused by disparities of the cut-off wavenumbers. In this work, we focus particularly on mercury cadmium telluride (HgCdTe), which is the most important material of IR cooled detector applications. Among the many advantages of this ternary alloy is the tunability of the bandgap energy with Cadmium composition, as well as the high quantum efficiency. In order to predict and understand spectral inhomogeneities of HgCdTe-based IRFPA, we propose a modeling approach based on the description of optical phenomena inside the pixels. The model considers the p-n junctions as a unique absorbent bulk layer, and derives the sensitivity of the global structure to both Cadmium composition and HgCdTe layer thickness. For this purpose, HgCdTe optical and material properties were necessary to be known at low temperature (80K), in our operating conditions. We therefore achieved the calculation of the real part of the refractive index using subtracti
Infrared Focal Plane Arrays (FPA) are increasingly used to measure multi- or hyperspectral images. Therefore, it is crucial to control and modelize their spectral response. The purpose of this paper is to propose a modeling approach, adjustable by experimental data, and applicable to the main cooled detector technologies. A physical model is presented, taking into account various optogeometrical properties of the detector, such as disparities of the pixels cut-off wavelengths. It describes the optical absorption phenomenon inside the pixel, by considering it as a stack of optical bulk layers. Then, an analytical model is proposed, based on the interference phenomenon occurring into the structure. This model considers only the three major waves interfering. It represents a good approximation of the physical model and a complementary understanding of the optical process inside the structure. This approach is applied to classical cooled FPAs as well as to specific instruments such as Microspoc (MICRO SPectrometer On Chip), a concept of miniaturized infrared Fourier transform spectrometer, integrated on a classical Mercury-Cadmium-Telluride FPA, and cooled by a cryostat.
Today, both military and civilian applications require miniaturized and cheap optical systems. One way to achieve this trend consists in decreasing the pixel pitch of focal plane arrays (FPA). In order to evaluate the performance of the overall optical systems, it is necessary to measure the modulation transfer function (MTF) of these pixels. However, small pixels lead to higher cut-off frequencies and therefore, original MTF measurements that are able to extract frequencies up to these high cut-off frequencies, are needed. In this paper, we will present a way to extract 1D MTF at high frequencies by projecting fringes on the FPA. The device uses a Lloyd mirror placed near and perpendicular to the focal plane array. Consequently, an interference pattern of fringes can be projected on the detector. By varying the angle of incidence of the light beam, we can tune the period of the interference fringes and, thus, explore a wide range of spatial frequencies, and mainly around the cut-off frequency of the pixel which is one of the most interesting area. Illustration of this method will be applied to a 640×480 microbolometer focal plane array with a pixel pitch of 17µm in the LWIR spectral region.
Recent developments in the Mid Wave InfraRed (MWIR) optical domain were made on materials, optical design and
manufacturing. They answer increasing demands for more compact, less temperature dependent optical systems with
increased optical performances and complexity (multi- or hyper- spectral imagery). At the same time, the
characterization of these components has become strategic and requires solutions with higher performance.
The optical quality of such devices is measured by wave front sensing techniques. PHASICS previously developed wave
front sensors based on Quadri-Wave Lateral Shearing Interferometry (QWLSI) using broadband microbolometers
cameras for infrared measurements. However they suffer from reduced light sensitivity in the MWIR domain, which
limits their use with broadband sources such as black bodies. To meet metrology demands, we developed an innovative
wave front sensor. This instrument combines the metrological qualities of QWLSI with the radiometric performances of
a last generation detection block (Infrared Detector Dewar Cooler Assembly, IDDCA) with a quantum infrared focal
plane array (IRFPA) of HgCdTe technology.
The key component of QWLSI is a specific diffractive grating placed a few millimeters from the focal plane array. This
requirement implies that this optics should be integrated inside the IDDCA. To achieve this, we take advantage of the
experience acquired from recent developments with optics integrated in IDDCA. Thanks to this approach, we developed
a high spatial resolution MWIR wave front sensor (160x128 points) with a high sensitivity for accurate measurements
under low-flux conditions.
This paper will present the instrument technological solutions, the development key steps and experimental results on
various metrology applications.
We present a compact real-time multispectral camera operating in the mid-infrared wavelength range. Multispectral images of a scene with two differently spectrally signed objects and of a burning solid propellant will be shown. Ability of real-time acquisition will thus be demonstrated and spectra of objects will be retrieved thanks to inversion algorithm applied on multispectral images.
A concept of Fourier-transform infrared spectrometer integrated on a focal plane array (FTIR-FPA) has been developed
for very fast acquisition of spectral signatures. The basic idea is to use the upper surface of the focal plane array as the
first mirror of a two-wave interferometer, which creates interference fringes directly inside the active layer. Two
technologies have been developed. In a "monolithic" version of our FTIR-FPA concept, the cavity is made by grinding
the substrate to the shape of a wedge. In a "hybrid" version, the cavity is made by hybridizing a Silicon plate just above
the focal plane array.
In this paper, we report the development of low flux short wavelength infrared radio-imaging
systems to study the radiance due to nightglow emission. This radiation is mainly due to the desexcitation of
hydroxyl molecules in the upper atmosphere. It is present in the visible range and reaches its maximum
value (at ground level) in the short wavelength infrared band between 1.4 and 1.8μm. The nightglow may be
an interesting additional light source for night vision systems in moonless or cloudy sky conditions. In this
paper, we describe the experimental setup and present first results of the measurement campaigns that we
performed at the Observatoire de Haute-Provence in France and at the European Southern Observatory site
of La Silla in Chile.
We will present a brief overview of the interest in subwavelength gratings for spectral filtering in the mid-infrared wavelength range. Guided-mode, plasmonic and dipolar resonances will be considered. We will particularly focus on components fabricated in our laboratories, achieving band-pass or cut-band filtering. Optical characterization will be shown, revealing resonances with high quality factors. Multispectral camera has been realized by integrating our components into a cooled infrared focal plane array.
A comprehensive approach to quantify more accurately the physical phenomena leading to blooming effects on infrared
sensors is proposed. Laser dazzling effects on mid-infrared HgCdTe focal-plane-array are investigated thanks to the use
of an experimental breadboard. This dedicated breadboard is capable of delivering intense laser spots on focal-planearrays
with accurate control of laser spot diameter, position, power and pulse time sequence. The main subsystems of
this experimental bench are described, laser sources, focusing optics, power control device and opto-electronic
synchronization. HgCdTe focal-plane-array dazzling experiments are then analyzed. The main parameters affecting
blooming are varied and the impact of spatial, time and energy characteristics on the resulting dazzled image are
assessed.
We have recently shown that dewar-level integration of optics is a promising way to develop compact IR cameras.
Indeed, the integration of optics into the dewar leads to simple and entirely cooled optical architectures dedicated to
imaging applications with large-field of view. Here, we review the optical elements we could add in those devices to
make a hyper- or multispectral imager. Among them, we find specific focal-plane arrays with a built-in spectrometry
function, plasmonic filters combined with a multichannel optical design, and birefringent interferometers. Several optical
architectures will be detailed with first experimental results.
A new configuration of micro-spectrometer based on an infrared stationary Fourier transform (FTIR)
interferometer has been developed at ONERA. Our device is based on a classic infrared focal plane array (FPA)
of HgCdTe technology with a built-in two-wave wedge-like interferometer. This new architecture generates
research works in several domains. Technological researches are conducted in collaboration with the CEA to
optimize the manufacturing process and reduce the technological defects. In parallel, researches in optical
design are conducted to implement the FTIR-FPA in a complete system. For this, theoretical work is needed to
understand and describe the fringes formation inside the detection structure when illuminated by a wave which is
not ideal, i.e. emitted by an extended source positioned at a finite distance from the detection plane. The results
of this theoretical study are presented. These results are exploited to design a compact spectrometer with a very
simple optical architecture. First experimental data are presented and discussed.
Today's infrared focal plane arrays concentrate in a small volume of typically 1 cm3 the results of three decades of
research in microelectronics and packaging. Several technological breakthroughs have already been achieved leading to
the development of infrared focal plane arrays (IRFPA's) for high-performances applications requiring spatial and
thermal resolution, also for low-cost and high-manufacturing volumes (technology of uncooled micro-bolometers). The
next step is to reduce the optics and make it compatible with the successful IRFPA's fabrication technology. This paper
presents some methods and technologies we are exploring for high-performance and small infrared systems. These
developments have led to a tool box of micro-concepts described by an optical function (imagery or spectrometry)
integrated in the vicinity of the IRFPA. For this, old optical concepts have been revisited (pinhole optics, Talbot effect)
and first demonstrations of original IRFPA-based micro-optical assemblies will be given.
Over the past decade, several technological breakthroughs have been achieved in the field of optical detection, in
terms of spatial and thermal resolutions. The actual trend leads to the integration of new functions at the vicinity
of the detector.
This paper presents two types of integrated optics in the cryo-cooler, close to the MCT (CdHgTe) infrared
detector array. The first one, for spectro-imaging applications, is a Fourier-transform microspectrometer on chip
(MICROSPOC), developed for very fast acquisition of spectral signatures. Experimental results will be
presented. The second one, for large field of view applications, illustrates the high potentiality of the integration
of advanced optical functions in the Dewar of MCT detectors.
KEYWORDS: Long wavelength infrared, Mercury cadmium telluride, Avalanche photodetectors, Monte Carlo methods, Ionization, Mid-IR, Avalanche photodiodes, Signal to noise ratio, Temperature metrology, Mercury
We report the performances of LWIR (λc = 9.0 μm at 80K) HgCdTe electron injected
avalanche photodiodes (e-APD). In these devices, the exponential gain curve, up to gains
equal to 23 at 100K, and the low excess noise factor close to unity (F ~ 1-1.25) are indicative
of a single carrier multiplication process, which is electron impact ionization. The dark
current is mainly due to a diffusion current at low reverse bias and tunneling currents at high
reverse bias. A Monte Carlo model has been developed for understanding the multiplication
process in Hg1-xCdxTe e-APDs. We find a good agreement between first simulation results
and experimental measurements of the gain and the excess noise factor in both MWIR (x =
0.3) and LWIR (x = 0.235) e-APDs at 80K. Furthermore, simulations do not show any heavy
hole impact ionization. This model which enables to perform phenomenological studies aims
at identifying the main physical and technological parameters that influence the gain and the
excess noise. In the present work, it is used to study the influence of the thickness of the ndoped
region on the gain and the excess noise factor. We found that F still decreases while the
thickness of the n- layer decreases. However, an optimum thickness of the n- layer exists
around 1μm in terms of gain-voltage characteristic.
Circular diffraction gratings (also called diffractive axicons) are optical components producing achromatic non-diffracting
beams. They thus produce a focal line rather than a focal point for classical lenses. We have recently
shown in the visible spectral range that this property can be used to design a simple imaging system with a
long depth of focus and a linear variable zoom by using and translating a diffractive axicon as the only component.
We have then adapted this principle for the mid-wavelength infrared (MWIR) spectral range and the
long-wavelength infrared (LWIR) spectral range. A LWIR low-cost micro-camera, called MICROCARD, has
been designed and realized. First images from this camera will be shown. Moreover a way to design a compact
MWIR micro-camera with moveable parts integrated directly into the cryostat will be presented.
There is an emerging demand for compact infrared instruments, imagers and/or spectrometers, integrated on
ground or air vehicles for spatial and spectral data collection. To reach this goal, technological barriers have
already been overcome, leading to the development of infrared focal plane arrays (IRFPAs) for high-performance
applications (megapixel format, bispectral technology) but also for low-cost and high-volume manufacturing
(technology of uncooled micro-bolometers). The next step is to reduce the optics and make it compatible with
the successful IRFPAs fabrication technology. This paper presents MULTICAM, a small cryogenic infrared
camera. This optical system is composed of multi-level arrays of microlenses integrated in the cryostat and
inspired from invertebrate compound eyes. First experimental results will be presented.
The evaluation of technological parameters is of primary importance for the detector industry, since it allows both to validate the fabrication process and to optimize the electro-optical characteristics of the detectors. By measuring the spectral response of detectors with a high resolution, it is possible to display specific optical effects. Using a radiometric model of the detecting architecture, we are able to understand their physical origins and to determinate some technological and optical parameters. We have developed a test bench which provides spectral responses of infrared detectors using a Fourier transform spectrometer. The principle of the test bench and the methodology used are detailed. Experimental results, as well as the associated radiometric model, are presented for a dedicated 320×240 MCT LWIR focal plane arrays (FPAs).
A test bench has been developed at ONERA in order to measure the spectral responses of infrared focal plane arrays (IRFPAs). This test bench can deliver hyperspectral cartographies with rather good resolutions (better than 16 cm-1) on large spectral ranges (from 1.3 μm to 20 μm). The principle of this test bench will be described. Using this technique, tests have been performed on a large format (640x512) IRFPA of quantum-well technology operating in the 8- to 10-μm spectral range. The prototype tested had several small defects that produce spectacular hyperspectral cartographies. To explain the hyperspectral structures observed across the array, an empirical model based on Fourier optics will be presented.
By measuring the spectral responses of infrared focal plane arrays (IRFPAs), one can extract at a given wavelength the cartography of the pixels responses, called the hyperspectral cartography. Recently, hyperspectral cartographies have been obtained from IRFPAs that exhibited small defects of substrate thickness. These defects produce Fizeau fringes across the FPA. By purposely amplifying this phenomenon during the process of realisation, one can easily generate a good approximation of a two-beam interferometer in the immediate neighbourhood of the FPA. Like a classic Michelson interferometer with tilted plane mirrors, this on-a-chip interferometer produces a spatially-modulated interferogram, the Fourier-transform of which yields the spectral content of the illuminating beam. A first prototype of this Fourier-transform microspectrometer on a chip (MICROSPOC) has been realised and tested. Experimental results will be presented and the potential of this approach will be discussed.
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